= Laser
{tag=Light source}
{wiki}
What makes lasers so special: <Lasers vs other light sources>.
\Video[https://www.youtube.com/watch?v=_JOchLyNO_w]
{title=How Lasers Work by Scientized (2017)}
{description=
An extremely good overview of how lasers work. Clearly explains the electron/photon exchange processes involved, notably <spontaneous emission>.
Talks about the importance of the metastable state to achieve <population inversion>.
Also briefly explains the imperfections that lead to the slightly imperfect non punctual spectrum seen in a real laser.
* https://youtu.be/_JOchLyNO_w?t=188 says <LED> is "also monochromatic", but that is not strictly true, it has way way larger frequency band than a laser. Only narrower compared to other sources such as <#incandescent light bulbs>.
* https://youtu.be/_JOchLyNO_w?t=517 <stimulated emission>. This is the key to laser formation as it produces coherent photons.
* https://youtu.be/_JOchLyNO_w?t=581 <spontaneous emission> happens too fast (100 ns), which is not enough time for <stimulated emission> to happen. <Metastable electrons> to the rescue.
* https://youtu.be/_JOchLyNO_w?t=832 the parallel mirrors select perpendicular photons preferentially
}
\Video[https://www.youtube.com/watch?v=saVE7pMhaxk]
{title=Laser Fundamentals I by <#Shaoul Ezekiel>}
{description=2008, <MIT>. Many more great videos in this series.}
Bibliography:
* https://phys.libretexts.org/Courses/University_of_California_Davis/UCD%3A_Physics_9HE_-_Modern_Physics/06%3A_Emission_and_Absorption_of_Photons/6.3%3A_Lasers His <Rate My Professors> is amazing: https://www.ratemyprofessors.com/ShowRatings.jsp?tid=1783467
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